Abstract
Chlorophylls are an essential class of cofactors found in all photosynthetic organisms. Upon absorbing a photon, the excited state energy of the chlorophyll can either be transferred to another acceptor molecule, or be used to drive electron transfer. When acting as the primary donor in the bacterial photosynthetic reaction center, light-induced charge separation results in the formation of a cationic bacteriochlorophyll dimer. The hyperfine interactions between the unpaired electron of the 15N labeled bacteriochlorophyll cation radical and its four pyrrole nitrogens are probed with X- and Q-band 15N HYSCORE spectroscopy in frozen solution. The powder-type HYSCORE shows the basic (να(β), νβ(α)) cross-features as well as several types of combination cross-features. The nitrogen tensors were resolved in the squared-frequency representation of the HYSCORE spectra, and simulations of the combination peaks allowed for further refinement of the hyperfine coupling constants. The nitrogen tensors were found to have coupling constants a=3.28 MHz, T=1.23 MHz (N1 and N2), a=4.10 MHz, T=1.25 MHz (N3), and a=4.35 MHz, T=1.70 MHz (N4). The combination features were assigned based on a linear regression analysis of the cross-ridges in the squared-frequency representation as well as spectral simulations. The methodology discussed here will provide an important foundation for analyzing and understanding complex two-dimensional spectra from several I=1/2 nuclei.
Dedicated to: Kev Salikhov on the occasion of his 80th birthday and in recognition of his seminal contributions to the theoretical foundations of pulsed EPR spectroscopy.
Acknowledgments
This research was supported by the DE-FG02-08ER15960 Grant from Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Sciences, U.S. Department of Energy (S.A.D.), and NCRR/NIH Grant S10-RR15878 and S10-RR025438 for pulsed EPR instrumentation. A.T.T. gratefully acknowledges support as a NIH trainee of the Molecular Biophysics Training Program (5T32-GM008276).
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